Antenna system for a radio telephone in a vehicle that has a primary antenna to realize telephone communication with optimal connection quality and a secondary antenna that will at least allow telephone communication with lower connection quality when the primary antenna becomes damaged. The primary antenna is advantageously located on the outside of the vehicle, e.g. on the surface of the body of the vehicle or near the window, which provides a higher level of performance for sending as well as for receiving, but which can lead to complete failure if the vehicle is involved in an accident. The secondary antenna serves as an emergency antenna after the failure of the primary antenna, the main antenna, especially after an accident, to establish a telephone connection in order to call for help. As a radio telephone is only ready for operation when all of its components are functioning properly and an antenna is particularly sensitive mechanically when placed on the exterior of a vehicle, the solution according to the invention significantly increases the reliability of a radio telephone in an emergency. The application of the invention is not limited to conventional radio telephones with one operating frequency band. The invention can also be applied to antennas utilizing various operating frequency bands for mobile communication.
Car phones are usually equipped with an external or window antenna. The location of this antenna is primarily determined by the requirements that need to be met to achieve optimal sending and receiving quality.
One disadvantage of selecting such a location is that the probability of damaging the antenna to the point of total failure is high when the vehicle is involved in an accident or when other external forces act on the antenna. In particular, these other external forces acting on external antennas include, for example, the intentional destruction of the antenna by a stranger or the breaking off of the antenna while passing under an obstacle with low clearance. It is not possible to establish a radio telephone connection to call for help after a traffic accident or vehicle malfunction, for example, after the total failure of the antenna.
To eliminate this shortcoming an emergency or back-up antenna is installed in a protected location such as the passenger compartment of the vehicle as stated in publication EP-A1-0 859 237. This secondary antenna is then used for sending/receiving operations after the external antenna used as the main antenna fails. Each antenna is connected to the radio telephone via a separate coaxial cable. The radio telephone has two separate antenna connections for this purpose.
To obtain the maximum transmission quality and to prevent interference during normal send/receive operations, the emergency antenna is not to be used while the main antenna is functioning. This means that the emergency antenna and the corresponding wire are only to be put into operation in an emergency by the manual or automatic initiation of an emergency call. To accomplish this, an emergency call button is activated or the air bag and/or seat belt mechanism controller sends a corresponding control signal to the radio telephone when triggered to switch the radio telephone to the secondary antenna connection.
In principle, there are various solutions used to switch the radio telephone to the emergency antenna:
In simple solutions, the initiation of an emergency call in the radio telephone will automatically force the radio telephone to switch to the connection for the emergency antenna regardless of whether or not the main antenna is still operational. One requirement for this to occur is that there must be a high probability that the emergency antenna and its separate antenna wire still function due to installation in a protected location.
However, malfunctions or damage to the antenna feed cable leading to the emergency antenna can arise when connecting the antenna or operating the vehicle that remain undetected because the emergency antenna is not used during normal operation. Under certain circumstances, this antenna may not function properly in an emergency. Additionally, its efficiency is generally lower that that of the main antenna due to its installation location. This may also lead to the inability to connect to the base station using the less powerful emergency antenna when the vehicle is in an unfavorable position although the connection could be made using an intact main antenna.
To avoid this shortcoming, radio telephones with several antenna connections and other accessories periodically perform a test procedure in which the antennas are operated alternately and checked to see if they are functioning properly. This can be done, for example, by comparing the signal strength of the signal received or, in accordance with publication EP 0 859 237 A1, by comparing the signal strengths of the signal supplied and the signal reflected back by the antenna wire.
In this manner, malfunctions and damage to the antennas and the wires will be detected and indicated, and the unit can quickly switch to a functioning branch of the antenna. The test procedure is also generally performed when an emergency call is triggered so that the unit only switches to the less powerful emergency antenna when the main antenna has failed due to breakage, for example.
However, one disadvantage of these two solutions is that the radio telephone requires several antenna connections for the separate branches of the antennas and that an antenna feed cable must be installed for each antenna. In addition, the radio telephone must have special test circuitry to automatically perform the test procedure in the latter solution. The requirements stated therefore result in additional undesired complexity.
An antenna switching circuit to selectively use the internal or external antenna of a radio telephone is known from publication DE 197 19 657 A1. In contrast to the solution according to the invention, the internal antenna is an antenna that is built into the radio telephone and the external antenna is an automobile antenna that is connected manually to the radio telephone via a flexible feed cable and an antenna connector plug. A switching device on the HF module of the radio telephone disconnects the internal antenna from the module and connects the radio telephone to a vehicle antenna as soon as the antenna connector plug is inserted into a radio telephone socket equipped with contact springs. The contact springs of the socket form a switch that supplies the switching unit with a corresponding control voltage. The known solution only allows for the manual changing of antenna connections. There is no automatic switching performed in an emergency.
Based on the shortcomings of the known solutions, it is the task of the invention to create an antenna system with several antennas for a radio telephone in a vehicle in which, with little or no effort, the unit only switches to an emergency antenna when the main antenna is not present any more.